Re: [PATCH v2] KVM: SVM: Fix detection of AMD Errata 1096

["Singh, Brijesh" <brijesh.singh@amd.com>]

On 7/16/19 6:56 PM, Liran Alon wrote:
> AMD Errata 1096:
> When CPU raise #NPF on guest data access and vCPU CR4.SMAP=1, it is
> possible that CPU microcode implementing DecodeAssist will fail
> to read bytes of instruction which caused #NPF. In this case,
> GuestIntrBytes field of the VMCB on a VMEXIT will incorrectly
> return 0 instead of the correct guest instruction bytes.
> This happens because CPU microcode reading instruction bytes
> uses a special opcode which attempts to read data using CPL=0
> priviledges. The microcode reads CS:RIP and if it hits a SMAP
> fault, it gives up and returns no instruction bytes.
> 
> Current KVM code which attemps to detect and workaround this errata have
> multiple issues:
> 
> 1) Code mistakenly checks if vCPU CR4.SMAP=0 instead of vCPU CR4.SMAP=1 which
> is required for encountering a SMAP fault.
> 
> 2) Code assumes SMAP fault can only occur when vCPU CPL==3.
> However, the condition for a SMAP fault is a data access with CPL<3
> to a page mapped in page-tables as user-accessible (i.e. PTE with U/S
> bit set to 1).
> Therefore, in case vCPU CR4.SMEP=0, guest can execute an instruction
> which reside in a user-accessible page with CPL<3 priviledge. If this
> instruction raise a #NPF on it's data access, then CPU DecodeAssist
> microcode will still encounter a SMAP violation.
> Even though no sane OS will do so (as it's an obvious priviledge
> escalation vulnerability), we still need to handle this semanticly
> correct in KVM side.
> 
> As CR4.SMAP=1 is an easy triggerable condition, attempt to avoid
> false-positive of detecting errata by taking note that in case vCPU
> CR4.SMEP=1, errata could only be encountered in case CPL==3 (As
> otherwise, CPU would raise SMEP fault to guest instead of #NPF).
> This can be a useful condition to avoid false-positive because guests
> usually enable SMAP if they have also enabled SMEP.
> 
> In addition, to avoid future confusion and improve code readbility,
> comment errata details in code and not just in commit message.
> 
> Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
> Cc: Singh Brijesh <brijesh.singh@amd.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> ---
>   arch/x86/kvm/svm.c | 42 +++++++++++++++++++++++++++++++++++-------
>   1 file changed, 35 insertions(+), 7 deletions(-)
> 

Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>

thanks